This invention relates to fluid-gauging systems.
In capacitive fluid-gauging systems, the level of fluid in a reservoir is determined by sensing changes in the value of a capacitor located within the container. The capacitor has two plates, separated by an air- gap which is filled or emptied as the depth of fluid changes, thereby altering its capacitance. An alternating electric signal is supplied to the capacitor, and its output, after rectification, is supplied to a suitable measuring circuit.
The diodes used to rectify the output of the capacitor are mounted in the fluid tank and are subjected to the temperature changes of the fluid and air above the fluid. Typically, the temperature coefficient of such diodes is about -2mV/.degree. C. Since the voltage drop across the diodes affects the output of the capacitor sensor as supplied to the measuring circuit, it is usually necessary to compensate for this. Compensation may be achieved by use of a temperature sensor mounted close to the diodes and with knowledge of the temperature coefficient of the diodes.
This method of compensation has the disadvantage of requiring the use of additional components leading to increased complexity and reduced reliability. Also, the voltage drop across the diodes will be affected by other factors such as ageing which cannot be compensated by use of a temperature sensor.